I use it to get the best OCs. From the bios I can't go above 244 fsb (wich is decent for a newcastle), but nTune tunes my rig up to 266 fsb, so evan better.
I use it like this: I saved the profile in wich he found my best memory performance (eg. 266 fsb and all the other timings and stuff) and I put this profile to start with a .exe that has the sole purpose to trigger the nTune.
I have another profile with the default settings, just in case I don't need a lot of speed any more.
To be clear, I use nTune only because my mobo doesn't want to go higher than 244 from the bios, oterwise I prefer the classic bios OC.